1. Field of the Invention
The present invention relates to x-ray diagnostics, and more specifically, it relates to techniques for utilizing x-rays to characterize a sample.
2. Description of Related Art
Excluding skin cancer, breast cancer accounts for one in every three cancer diagnoses in the United States. As the leading cause of death in women age 35-50 in the U.S., breast cancer is a major public health problem, with 1 in 8 women expected to be diagnosed sometime during their lives. The National Cancer Institute has therefore designated breast cancer as a top research priority. X-ray mammography, based on absorption contrast, is currently the primary technique for the screening of breast cancer. Absorption contrast works well in distinguishing between hard tissue, bones and teeth, and soft tissues. In mammography, however, there is a need to distinguish between different types of soft tissue representing the malignant tissue and the normal tissue. As a consequence conventional absorptive mammograms have a high false positive rate, as high as 19 in 20, with up to 15% of patients with tumors not displaying irregular radiographs. The passage of x rays through a sample can be described by a complex index of refraction, n=1−δ−iβ, where δ represents a phase shift and β represents absorption. For x-ray energies of 15-25 keV, typical of mammograms, the phase shift term can be up to 1000 times the absorption term. As x rays pass through the sample, the different soft tissue results in different phase shifts, thereby causing distortion of the wave-front and refraction of the x rays. A phase sensitive approach, as opposed to a pure absorption technique, can result in a lower dose to the patient, by going to higher x-ray energies and can result in a significantly less false positive ratio as the diagnostic would directly measure the malignant tissue and have a significantly high sensitivity due to the three orders of magnitude higher phase shift term as opposed to the absorption term.
Conventional x-ray absorptive radiography is also used in a broad range of scientific areas including materials science, high energy density physics and Inertial Confinement Fusion (ICF) to name a few. In ICF, low Z materials are compressed to create the high density and temperature conditions necessary for fusion to occur. This is another area where contrast is difficult to achieve with conventional absorptive radiography. In high energy density physics experiments and non-destructive testing, phase sensitive diagnostics would enable the study of low Z material mixing and material/plasma interpenetration experiments without the need to add high Z elements for contrast, the presence of which can change the physical properties of the experiment through radiative cooling. As with mammography, the inherent disparity between the magnitude of the phase shift and absorptive terms would enable substantial increases in the sensitivity of these phase sensitive techniques over conventional absorptive radiography.
Several phase sensitive x-ray mammography systems have been proposed for providing a higher sensitivity, a lower rate of false positives and a lower x-ray dose to the patient than current devices. Techniques used in such systems include interferometry, diffraction-enhanced imaging, phase contrast imaging and Moire deflectometry. These techniques have been unsuccessful for a variety of reasons.
Several techniques have been ported over from the optical regime into the hard x-ray regime to enable phase-sensitive measurements. Current techniques for phase-sensitive x-ray imaging involve x-ray interferometry, diffraction enhanced imaging, phase contrast and Moire′ deflectometry techniques. Each of these techniques has limitations which prevent their widespread use for quantitative phase-sensitive x-ray imaging. X-ray interferometry, shown in FIG. 1A, utilizes three crystals to separate and recombine two hard x-ray beams to form an interference pattern on a detector. Synchrotron produced x-rays 10 are redirected by a monochrometer 12 and then are separated by a splitter 14, which separates the beam into two beams 16 and 18, which are redirected by mirror 20 to an analyzer 22 which then directs the beam to a detector 24. A sample 26 is placed in beam 16. Interferometry with hard x rays requires high temporal and spatial coherence and a very bright source to overcome the inefficiency of the crystals. Such techniques must use synchrotron sources. In addition, this interferometer is constructed from a single silicon crystal which greatly limits the size of the sample that can be studied. Diffraction-enhanced imaging, as shown in FIG. 1B, utilizes the rocking curve of a crystal as a filtering technique. Synchrotron produced x-rays 30 are redirected by a first monochrometer 32 and a second monochrometer 34 through a sample 36 and then are directed by and analyzer 38 to a detector 40. To quantitatively reconstruct the phase of an object requires many measurements at different analyzer angles to provide measurements of the entire field. It also requires a bright source due to the inefficiency of the crystals used as the analyzer. Phase contrast techniques, as shown in FIG. 3C, operate by taking images of the illuminated object at multiple distances from the target and iteratively reconstructing the phase of the object numerically. This technique requires measurements at multiple distances to reconstruct the various spatial scales in the object and is therefore not suitable for single shot operation. In FIG. 1C, synchrotron or micro-focus produced x-rays 50 pass through a sample 52, and onto a detector 54. Moire′ deflectometry, as shown in FIG. 1D, can only reconstruct the phase of the x-ray wave-front in one dimension. To reconstruct the full field would require multiple measurements, again inconsistent with single shot operation. In the figure, synchrotron or micro-focus produced x-rays 60 pass through a sample 62, a phase grating 64, an absorption grating 66 and onto a detector 68.
Practical phase sensitive mammography and x-ray diagnostic systems that overcome the problems inherent in prior art systems are desired.